1. Field of the Invention
This invention relates to a zoom lens system, and more particularly to a zoom lens system which has a large zooming ratio, a large diameter and a wide angle of field of view. In particular, the present invention is concerned with correction of the distortion of the zoom lens.
2. Description of the Prior Art
A variable magnification lens system is called zoom lens. The conventional zoom lens systems are suffering from the defect that the distortion cannot be completely removed, particularly in case that the whole size of the zoom lens system is made compact. Therefore, it has been very difficult to provide a zoom lens system of small size having a zooming ratio of not less than 10, a wide angle of view of not less than 60.degree., a large lens diameter and a large range of focusing capable of focusing an image of an object at a short distance of not more than 1 m. In a zoom lens system which satisfies the above conditions suffers from a distortion fluctuating from -5% to 30 5% as shown in U.S. Pat. No. 3,705,759.
The problem of the distortion will hereinbelow be described in detail with reference to FIG. 1. FIG. 1 is a longitudinal view of a conventional zoom lens system for explaining the above described problem. The above mentioned problem that the distortion increases when the angle of view is made larger than 60.degree. at the wide angle side in a zoom lens system which has a large diameter and large range of variation of magnification and is capable of focusing up to short distance is explained as follows. In order to correct aberrations for both the long distance and the short distance in the telescopic side of the zoom lens having a large diameter and a large range of variation of magnification, the shape of the lens group having a large diameter which is sensitive to the aberrations in the telescopic side is almost automatically determined. Therefore, the degree of freedom in design of the lens system for correction of the distortion in the wide angle side is very limited. Thus, the distortion increases as the size of the whole zoom lens system is made small in the above described type of zoom lens system.
With reference to FIG. 1, the diameter of the light flux is as large as the effective diameter of the zoom lens system as shown by C or C' in the telescopic side, i.e. narrow angle side. Therefore, in this stage the shape of the lenses has a great effect upon the spherical aberration. On the other hand, in the wide angle side, the light flux has a small diameter as shown by A or A'. The light flux A which advances along the optical axis of the zoom lens system is not affected by the shape of the lenses, but the light flux A' which advances obliquely with respect to the optical axis of the zoom lens system is greatly affected by the lens shape. Particularly, since the diameter of the lens system is very large in this type of the lens system, the light flux A' could have a large angle with respect to the optical axis. Therefore, the zoom lens system involving light fluxes inclined at such large angles with respect to the optical axis is very sensitive to the distortion. The spherical aberration, on the other hand, can be well corrected in the telescopic side if the refractive power of the lens L1 is made small, the refractive power of the lens L3 is further increased and the shape of the lenses L1 and L3 is made concave to the object side, which however all deteriorate the distortion.
Further, all the conditions for making the diameter of the lens system large, making the zooming ratio large and making the size of the lens system small result in enlargement of the diameter of the light fluxes C and C' in the telescopic side which results in increase in the negative distortion. In the wide angle side end, however, the negative distortion can be corrected independently of the correction thereof in the telescopic side, since the oblique light flux A' passes through the lens surface at different position from that at which the marginal light rays of the large light flux C passes and the light flux A' is highly sensitive to distortion.
However, even if the negative distortion can be corrected in the wide angle side end while controlling the distortion in the telescopic side as mentioned above, a positive distortion remains greatly in the wide angle side. The light flux B' passing the marginal portion of the image on the focal plane on which the positive distortion is maximized passes through almost the same position of the lens system as the marginal light rays of the light flux C advancing along the optical axis of the lens system in the telescopic side. Further, the angle of view of the light flux B' is small. Therefore, the light flux B' does not have a great effect upon the distortion. Accordingly, it is difficult to correct the positive distortion while sufficiently controlling the aberration in the telescopic side.